Multilayer thin films and thin film pigmented coatings for use in infrared and visual signature control, as well as solar heat load control (herein referred to generically as "signature control"), are in particular demand.
Such multilayer thin films are conventionally produced by depositing on a substrate, e.g., by vapor deposition or sputter deposition, a plurality of thin alternating layers of at least two optical materials having different indices of refraction and a predetermined differential between their indices of refraction. The larger the differential between the indices the greater the transmission or reflectance characteristic, and the broader the band of transmission or reflectance.
Conventional optical materials have indices of refraction in the order of from about 2 to about 4, which places an inherent limit on the differential between the indices, with a differential of about 2 being considered quite large.
Due to the limitations on the differential, it is necessary, in order to attain the desired degree of transmission or reflectance, i.e., the desired optical density, to build up a great many alternating layers of the two materials, e.g., 15 to 30 or more layers.
The performance of a multilayer optical device is wave band specific. It is controlled by the thicknesses and the number of the respective layers, and is dependent upon the refractive index difference between layers for the magnitude of its effect. If the thickness or the differential is changed, the performance changes. What is effective in the visible wave band may be of no use in the far infrared.
Materials useful in the production of such devices are elements from a small fraction of the periodic table, because most of the elements in the table are unsatisfactory from either performance or feasibility characteristics. Therefore, the same material may earn mention in many disclosures, but in far different contexts. The same materials, produced in different thicknesses and evaluated at other wave bands, may offer special, beneficial and unexpected results.
It is in the context of other domains and unique requirements that the present invention resides.